Apparatus and method for prestressing a countersunk fastener hole

ABSTRACT

A pull gun (30) is used for pulling a mandrel (32) through an expandable sleeve (126) positioned in a fastener hole (134, 136). The mandrel (32) expands the sleeve (126) as it moves through the sleeve (126), causing the sleeve (126) to prestress the metal which surrounds the hole (134, 136). The pull gun end of the hole is countersunk (138). The pull gun (30) includes an axially split ring (74) having a conical insert (90) which snugly fits within the countersink (138). The sleeve (126) extends through a center opening (80) in the expandable ring (126). Mandrel movement through the portion of the sleeve (126) within said central opening (80) forces the segments (76) of the ring (74) radially outwardly, causing the portions (90) of such ring (74) which are in the countersink (138) to expand and compress the metal which immediately surrounds the countersink (138). A solid film lubricant (127) is provided on the inner surface of the expandable sleeve (126). The coefficient of friction of such lubricant (127) and the contact area (66) of the mandrel (32) with the interior of the sleeve (126) are controlled for the purpose of providing a reaction force which is sufficient to generate enough pressure between the conical insert (90) and the walls of the countersink (138) to adequately compress the metal bounding the countersink (138).

DESCRIPTION

1. Technical Field

This invention relates to a method and apparatus for prestressing bothcountersink and straight portions of a countersunk fastener hole in asingle operation.

2. Background Art

In the prior art, holes which were to have countersinks were prestressedas a straight hole and then the hole was countersunk. In this type ofoperation the holes were countersunk after being prestressed. Thismethod of countersinking required one additional step that is eliminatedby the present invention.

In addition, the prior art method of countersinking after prestressinglimits the prestressing and the resulting improvement of fatigueproperties of the countersink to the region of the prestressing of thestraight hole; that is, prestressing before countersinking extendsradially only as far as the prestressing of the straight hole. Anotherproblem is that reworking of a hole which has been countersunk accordingto the prior art method, requires that the hole be drilled to anoversized condition with subsequent prestressing, reaming, countersinkmachining, and installation of an oversized fastener.

The basic patent relating to prestressing or cold expansion of fastenerholes, by use of a lubricated spacer sleeve, is my U.S. Pat. No.3,566,662, granted Mar. 2, 1971, and entitled Coldworking Method andApparatus. A method and apparatus for making the sleeves is disclosed byU.S. Pat. No. 3,665,744, granted May 30, 1972, to Clair M. Harter, andentitled Method And Apparatus For Making Sleeves. U.S. Pat. No.3,892,121, granted July 1, 1975, to me, Horace E. Hill and Joseph L.Phillips, and entitled Apparatus For Cold-Working Holes, discloses aform of mandrel and mandrel pulling tool or gun. My U.S. Pat. No.4,187,708, granted Feb. 12, 1980, and entitled Pulling Apparatus AndMethod, discloses a preferred form of pull gun.

The aforementioned patents, the references cited against them, and apaper by Joseph L. Phillips entitled "Fatigue Improvement By SleeveColdworking", should be carefully considered for the purpose of puttingthe present invention into proper perspective relative to the prior art.

DISCLOSURE OF THE INVENTION

The present invention provides an apparatus and method by which apreviously drilled countersunk hole is prestressed in the region of thehole and the region of the countersink in a single one-sided operation.

The present invention requires only two process steps--the first beingthe drilling of the hole and the countersink in a single operation. Thesecond is the prestressing or cold expansion of the hole and thecountersink in a single one-sided operation. This method, and apparatusprovided therefor, improve the fatigue properties in the region of thecountersink and the hole and eliminate the separate machining of thecountersink required by the prior art.

Accordingly, it is an object of the invention to provide optimum fatigueproperties in a hole having a countersink by prestressing thecountersink area in the same operation in which the hole is prestressed.The region of compressive residual stresses, according to the invention,extends radially outwardly from the outer edge of the countersink for adistance equal to the radial thickness of the prestress region of thehole. This is in contrast to the prior art wherein the extent ofprestressing of the countersink is limited to the actual prestress ofthe straight hole.

It is another object of the invention to reduce the costs of thefastener hole drilling, countersinking, and prestessing in contrast tothe prior art systems. The present invention eliminates the separatecountersinking operations. The costs are also less than the priorinterference fit fastener systems, because of reduced tolerances in thedrill/countersinking operation, reduced inspection requirements, andbecause of the low interference, 0.0005 inch, with a fastener that doesnot require the driving operation employed with most high interference,0.004 inch, fasteners.

It is a still further object of the invention to provide a prestressingsystem which reduces the susceptibility to stress corrosion byincreasing the area of compressive residual stress around the edge ofthe countersink.

It is another object of the invention to provide prestressed countersinkholes in which rework operations are less costly because therequirements to ream and re-countersink the hole are eliminated. Inaddition the low interference of the installed fastener facilitateseasier removal when necessary, and thereby minimizes hole damage.

It is still another object of the invention to provide an improvedmethod and apparatus for prestressing countersunk holes in which thelength of the maximum diameter part of the mandrel and the coefficientof friction of the lubricant used within the sleeve are controlled as away of controlling the reaction force which maintains the jaws loadedagainst the surface of the countersink.

Further objects and advantages of the invention may be brought out inthe following part of the specification wherein small details have beendescribed for the competence of disclosure, without intending to limitthe scope of the invention which is set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the accompanying drawings, which are for illustrativepurposes:

FIG. 1 is a longitudinal sectional view of a section of a workpiece,showing a countersunk fastener hole which has been prestressed with aprior art technique;

FIG. 2 is the same type of view as FIG. 1, but showing the region ofcompressive residual stress which results from prestressing inaccordance with the present invention;

FIG. 3 is a fragmentary isometric view of the nose of a pull gun and amandrel embodying features of the present invention;

FIG. 4 is an exploded isometric view of the nose portion of the pullgun, the mandrel, and the prestressing sleeve;

FIG. 5 is the first of a series of four similar longitudinal sectionalviews illustrating the sequence of the prestressing operation accordingto the invention, the first view showing the nose portion of a pull gun,a portion of a workpiece immediately surrounding the hole andcountersink to be prestressed, a prestressing sleeve in section, and themandrel in side elevation, said first view showing the position of theprestressing tools immediately following the insertion of the mandreland sleeve into the fastener hole;

FIG. 6 is a view similar to FIG. 5, illustrating the retraction of themandrel by the pull gun, the increasing diameter portion and maximumdiameter portion of the mandrel having commenced prestressing a portionof the workpiece, and the jaws of an expandable ring being forced intothe countersunk portion;

FIG. 7 is a view similar to FIGS. 5 and 6, illustrating the mandrel inthe process of exerting a generally radially outwardly directed force onthe jaws of the expandable ring to prestress the countersink;

FIG. 8 is a view similar to FIGS. 5-7 illustrating the mandrel in afully retracted position at the completion of the prestressing of thecountersink;

FIG. 9 is a longitudinal sectional view of the countersunk fastenerhole, showing a ridge formed in the cylindrical portion by a splitsleeve and similar ridges formed by the countersink prestressing ringjaws; and

FIG. 10 is an end view looking axially towards the fastener hole fromthe countersunk side, taken generally from the aspect of line 10--10 inFIG. 9.

BEST MODE OF THE INVENTION

Referring again to the drawings, FIG. 1 is a view of a prestressedfastener hole 10 according to the prior art. The prestressing isindicated in the annular shaded area 12 in the workpiece 14. In theaircraft industry such a workpiece may typically be aluminum, titanium,or steel.

In the prior art, the prestressing occurred before the hole 10 wasreamed, and countersunk at 16. The prestressing effect or compressiveresidual stresses around the hole extends radially the extent of oneradius, for example, from the inner wall into the workpiece. Becausecountersinking occurred after prestressing, the amount of prestressingin the countersink region is reduced relative to that of the remainderof the hole. For example, at the outer edge of the countersink theprestressing extends substantially less than the length of the radius ofthe hole.

FIG. 2 shows a workpiece 20 in which a fastener hole 22 and acountersink 24 have been drilled in a single operation. Thereafter thearea around the fastener hole and countersink was prestressed accordingto the invention so that the prestress radially outwardly the holeextends to the distance of one radius, for example, and similarly theprestressing of the countersink also extends a distance equal to oneradius, for example.

In FIGS. 3 and 4 devices for prestressing a fastener hole and itscountersink are shown in detail. A pull gun, generally designated as 30,is shown fragmentarily. Pull gun 30 may be of the type disclosed in myU.S. Pat. No. 4,187,708, granted Feb. 12, 1980.

The pull gun 30 is operated hydraulically to move a mandrel 32 from anextended, FIGS. 3 and 5, to a retracted position, FIG. 8. The hydraulicoperating means is disclosed in detail in U.S. Pat. No. 4,187,708.

A nose cap 34 is provided at the working end of the pull gun. Nose cap34 comprises a cylindrical side wall and a radial end wall 38 formed toinclude a cylindrical center opening 36. The end portion of nose cap 34opposite end wall 35, includes internal threads 40 adapted to bethreadedly engaged with external threads 42 of a jaw retainer nut 44having the same general configuration as the nose cap 34. Nut 44includes an end wall center opening 46 through which the mandrel isreciprocated.

The mandrel 32 is threadedly engaged at its inner end 54 to an adapter56 which is reciprocated by the hydraulic means, as described in theaforementioned U.S. Pat. No. 4,187,708. A boss 58 on the mandrel 32 isin abutment with the adapter 56 when the mandrel 32 is tightlythreadedly engaged therein.

Outwardly of the boss 58 is a small diameter or intermediate cylindricalportion 62 of the mandrel 32 and extending outwardly therefrom is anincreasing diameter, truncated conical portion 64. The portion 64terminates in a maximum diameter cylindrical portion 66, having auniquely long axial length about equal to its diameter. The maximumdiameter of the mandrel is slightly smaller than the diameter of thestarting hole to be prestressed (e.g. smaller by an amount about equalto three times the sleeve wall thickness). The outer end of the mandrelis formed by a truncated conical decreasing diameter portion 70.

A generally cylindrical ring 74 is split along axial sections into sixequal size segments 76 which when positioned together along their splitsurfaces have a generally cylindrical, axially directed passage 80extending therethrough. The ring parts 76 have a circumferential annulargroove 82 in which there are elastic rings 86 positioned to bias thesegments 76 together. An outer annular surface 88 of the ring 74 slopesinwardly and terminates in an axially outwardly directed jaw formingportion 90 of each segment. The surface 88 is normally in partialcontact with an annular shoulder 38 on the outer inside of the nose cap34.

The inner surfaces of the ring sections 76 are segments of acylindrical. Together they form the passage 80. The outer ends of thejaw portions 90 are in the nature of conical surfaces 94 with theirlargest diameter being axially inwardly and terminating in cylindricalsurfaces 96 which in turn terminate in the annular surface 88. Innerannular surfaces 100 on the ring 74 are generally perpendicular to thesurfaces forming the cylindrical passage 80.

A second generally cylindrical, split ring or disk 102 is formed of sixsegments 104, FIGS. 4-8. Along the split surfaces the ring portions 104are adapted to fit together to form a substantially cylindrical ringhaving an axially directed, generally conical passage 110 therethrough,the conical diverging inwardly so that the larger end of the passage 110is at the inner end of the ring 102. The outer annular surfaces 114 ofthe ring 102 are normally in abutment with the inner annular surfaces100 of the ring 74. Inner annular surfaces 116 are normally in contactwith the annular surface 50 of the inner cap 44. The ring 102 has acircumferential annular groove 118 with an elastic ring 120 therein tobias the ring segments 104 and 106 together. The diameters of thepassage 110 are smaller than the diameter of the cylindrical passage 80so as to provide annular shoulders 122 on the inner circumferentialsurface of the annular surface 114.

The segments of the rings 74 and 102 are limited in axial movement tothe extent provided by the acute angle formed radially inwardly by thesurface 38 of the cap 34 and the surfaces 88 of the outer ring, FIGS.5-8.

When the mandrel 32 is extended, FIGS. 3 and 4, a thin axially split,cold-expansion or prestressing steel sleeve 126 is slipped over theouter end of the mandrel onto the small diameter portion 62. Such acold-expansion sleeve has a comparable or higher modulus and yield thanthe material of the workpiece. The interior surface 127 of the sleeve126 has a solid film lubricant of a commercial type which includes leadoxide, graphite, and molybdenum disulfide. This lubricant film iscapable of withstanding more than 400,000 psi. See the aforementionedU.S. Pat. No. 3,566,662, and U.S. Pat. No. 3,665,744, granted May 30,1972, to Clair M. Harter, for detailed descriptions of the sleeves and amanner of constructing them. Also see the aforementioned articleentitled "Fatigue Improvement By Sleeve Coldworking" for a discussion oflubricant selection. The preferred lubricant comprises molybdenumdisulfide, graphite, a binder, a solvent (e.g. toluol) and possibly somelead oxide. As is known to those skilled in the art, the coefficient offriction of this lubricant can be varied by changing the variouscomponents.

In FIGS. 5-8, the prestressing operation of the invention isillustrated. Two abutting workpieces 130 and 132, adapted to be securedtogether by a fastener, have holes 134 and 136, respectively, drilledtherethrough. The hole 136 has a countersink 138 which typically is madein the same operation as the hole 136, the countersink having the sameangle as the surfaces 94 of the jaws 90.

In FIG. 5, the mandrel 32 is in the fully extended position relative tothe gun 30 and has been moved through the holes 134 and 136. The sleeve126 is positioned on the small diameter portion 62 of mandrel 32. Theouter end surface 35 of the nose cap 34 is spaced from the workpiecesurface 140.

When the mandrel 32 is in the fully extended position, FIG. 5, the outersurface of the boss 58 is in contact with the surfaces 116 of thesegments of the ring 102 so that the segments of both rings are tiltedwith respect to the gun and the workpiece. The surfaces 88 are in fullcontact with the shoulder 38 of the nose cap 34 and the conical surfaces94 of the jaws are in partial contact with the countersink 138. Theshoulder area 122 of the surfaces 114 of the ring 102 are in contactwith the inner end of the sleeve and the cylindrical surface 80 of thering 74 engage the outer surface of the sleeve but are tilted away fromthe sleeve as they extent axially outwardly.

Because the prestressing process is a one-sided operation, the splitsleeve 126 is installed on the mandrel 32 prior to inserting the mandrel32 and sleeve 126 into the hole to be expanded and prestressed. Aclearance between the sleeve and the wall of the hole is necessary andis equal to approximately 0.003 inch. As may be visualized from FIG. 8,to overcome this clearance, it is necessary for the beginning of thetapered section 64 of the mandrel to travel axially, after contact,approximately 0.065 inch before starting to expand the hole 134.

As shown in FIG. 6, the mandrel has been retracted substantially withrespect to the work piece 130, and the increasing diameter portion 64and the maximum diameter portion 66 have prestressed the area around thehole 134 as indicated by the broken lines 144 and 146, respectively. Inthe axial movement of the mandrel, the diameter of the sleeve 126 isstretched and the prestressed area is compressed as shown. In the axialmovement prior to that indicated by FIG. 6, the surfaces 116 of the ring102 had moved into abutment with the shoulder 50 on the inner cap. Thesurfaces 100 and 114 of the respective rings had moved into abutment soas to be at right angles to the axis of the mandrel. The cylindricalsurfaces of the ring passage 80 had moved into contact with the sleeveand the conical surfaces 94 of the jaws 90 had moved so as to fullyengage the countersink 138 but with no radial pressure. During theprestressing of the surfaces as indicated at 144 and 146 by the radialexpansion of the sleeve, the shoulders 122 fully restrain the sleeveagainst any axial movement.

As shown in FIG. 7, the ring members remain axially in the samepositions as in FIG. 6, and as the mandrel is retracted, the changingdiameter portion 64 stretches the hole portions as it passestherethrough. In FIG. 7 mandrel 32 is in a position where theprestressing of the cylindrical hole portions has been completed. Whenthe changing diameter portion 64 of the mandrel is moved so as to be inradial alignment with the jaws 90, the sleeve is further stretched so asto move the ring segments 76 of the ring 74 radially outwardly, FIG. 7,so that the conical surfaces 94 of the jaws 90 move into a high pressureprestressing contact with the countersink surface 138. The verysubstantial compressive force exerted thereagainst prestresses thecountersink region, as shown in FIGS. 7 and 8.

In FIG. 8, the maximum diameter portion 66 has moved out of the holeportions and has stretched the entire sleeve so that the inner end is incontact with the cylindrical surfaces 80 of the ring 74. In FIG. 8, themaximum diameter portion 66 continues to exert radial force on the ringso as to continue the stretching of the countersink until it movesalmost totally out of the inner end of the sleeve. At this point theprestressing of the couuntersink has been completed to the extent shownin FIG. 2, where the prestressing on the countersink is the samedistance from the surface thereof as is the distance of the prestressingfrom the wall surface of the hole, in contrast to the prior artprestressing shown in FIG. 1.

A considerable pulling force is required to pull the mandrel through thesleeve and cold-expand or prestress the hole and countersink. Themaximum diameter cylindrical portion 66 of the mandrel increases thefriction and pulling forces during the prestressing operation around thehole. For 3/16"-1/2" diameter holes in various aluminums these forcesare in the range of 2,000 to 9,000 lbs. This same pulling force on themandrel reacts against the jaws 90 and this reaction prevents thesurfaces 94 from sliding out of the countersink during the prestressingoperation. The jaws 94 are held axially in the countersink by means ofthe positioning of the two rings 74 and 102 between the shoulders 38 and50 and the arrangement of the sleeve on the shoulders 122, but thisarrangement does not cause any great prestressing force against thecountersink. According to an aspect of the invention, the coefficient offriction of the lubricant within the sleeve and the axial length ofmandrel portion 66 are controlled in order to obtain an effectivereaction force. Preferably, the length of mandrel portion 66 issubstantially equal to the diameter at mandrel portion 66.

As stated, there is a minimum of 0.003 inch clearance between theoutside of the sleeve and the wall of the hole for the larger diameterportions of the mandrel to enter the hole and sleeve. The taper of themandrel increasing diameter portion 64 is 0.045 inch/inch and it has totravel 0.022 inch for each 0.001 inch of the aforesaid clearance or aminimum of 0.066 inch travel. In the action depicted in FIGS. 7 and 8,there is no clearance between the surfaces 94 and the countersink.Therefore, part of the countersink and part of the hole are expandedsimultaneously so as to result in a required higher pulling force. Thisincreases the prestressing through a coining effect.

Another feature that helps maintain the jaw surfaces 94 on thecountersink, is that the outside of the sleeve is not lubricated. Itfirst locks itself in the wall of the hole when the mandrel starts toexpand the hole, and the friction between the outside of the sleeve andthe jaws also provides a force to contain the segmented jaws in thecountersink. As previously stated, the coefficient of friction of thelubricant within the sleeve is controlled as a further way ofmaintaining the jaw surfaces 94 on the countersink.

In FIGS. 9 and 10, the workpiece 132 is shown after the hole andcountersink have been prestressed. That is, when the sleeve and mandrelhave been removed, the hole 136 has been expanded to be as 136A and thecountersink has been expanded to be as 138A. As the split sleeve 126 isexpanded in the hole, its diameter is stretched and the split iswidened. The presence of the split results in formation of a small ridge156. Similarly, the splits between the jaw surfaces 94 create very smallmarks 158 in the countersink. The marks in the countersink are of nosignificance to the rivets or bolts fit into the fastener holes. In someinstallations it may be desirable to remove the ridge 156 by broachingor reaming, but it is now believed that resizing broaching isunnecessary.

The invention and its attendant advantages will be understood from theforegoing description and will be apparent that various changes may bemade in the form, construction, and arrangement of the parts of theinvention without departing from the spirit and scope thereof orsacrificing its material advantages, the arrangements hereinbeforedescribed being merely by way of example. I do not wish to be restrictedto the specific form shown or uses mentioned except as defined in theaccompanying claims.

I claim:
 1. In a pull gun of a type which is used for pulling a mandrelthrough a split prestressing sleeve located within a fastener hole, sothat an increasing diameter portion and an adjacent maximum diameterportion of the mandrel can exert expansion forces on the sleeve, tocause the sleeve to be expanded in the radial direction and prestressingthe metal around the fastener hole, to in that manner improve fatigueproperties of the metal, the improvement comprising:an axially split,segmented expandable ring member having a central passageway throughwhich the mandrel travels as it is being retracted into the pull gun;said passageway being normally smaller in diameter than the maximumdiameter of the mandrel and a substantial part of the increasingdiameter portion of the mandrel, so that as the mandrel moves throughsuch passageway it enlarges the passageway by forcing the segments ofthe ring member apart; and said ring member segments including means toengage a countersunk portion of said hole to prestress metal forming thesame when said segments are forced apart in response to axial movementof the mandrel through said expandable ring member.
 2. Apparatusaccording to claim 1 in which:said means to engage are conical shapedjaw portions having substantially the same angle as said countersunkportion.
 3. Apparatus according to claim 1 in which:said maximumdiameter portion has an axial length about equal to its diameter. 4.Apparatus according to claim 1 in which:said means to engage are jawportions which together form a segmented annular jaw sized to fit withinthe countersunk portion.
 5. Apparatus according to claim 1 in which:saidincreasing and maximum diameter portions enlarge said passageway in saidexpandable ring member to force said segments apart outwardly wherebysaid countersunk portion is stretched and prestressed.
 6. Apparatusaccording to claim 4 in which:said expandable ring member is fitted inone end of said gun, said end being the end through which said mandrelis retracted; said jaw portions extending outwardly from said one end; asecond expandable member in said gun inwardly of said ring member andfitted around said mandrel; said second member having inner surfacesthereof in abutment with a gun surface and said ring member being infixed axial abutment with said second member when said annular jaw isprestressing said countersunk portion.
 7. Apparatus according to claim 6in which:an axial inner portion of said sleeve on said mandrel extendsthrough said passageway in said ring member; and means on secondexpandable member extending adjacent and around said mandrel to supportsaid inner end of said sleeve against inwardly axial movement when saidmandrel is being retracted during the prestressing operation. 8.Apparatus according to claim 7 in which:said second member is an axiallysplit disk having a passageway through which said mandrel extends; saidpassageway through said second member being generally conical and havingits smallest diameter toward said inner end of said sleeve; said meanson said second member extending generally radially from said smallestdiameter along a surface of said second member in abutment with saidring member; said second member radial surface providing a shoulder tosupport said inner end of said sleeve.
 9. A method of prestressing acountersunk fastener hole or the like in a metal member,comprising:positioning a prelubricated cold-expansion sleeve on areduced diameter intermediate portion of a cold-expansion mandrel, saidintermediate portion being axially inwardly of an increasing diameterportion, a maximum diameter portion of said mandrel extending outwardlyof said increasing diameter portion; inserting the mandrel and sleeveinto the hole from its countersunk side; holding the sleeve in the holewhile retracting the mandrel through the sleeve and through an axiallysplit ring having an axially split annular prestressing portionprojecting into the countersink, so that as the mandrel is retracted theincreasing diameter and maximum portions of the mandrel expand thesleeve radially to prestress the metal surrounding said hole, and expandthe ring and annular portion radially to prestress the metal surroundingthe countersink.
 10. The method of claim 9, wherein the sleeve is heldin the hole by contacting its inner end with an axially split disk. 11.The method of claim 9, comprising controlling the friction of themaximum diameter portion of the mandrel within the sleeve to in thatmanner produce a reaction force between the mandrel and the holesufficient to maintain the annular portion of the axial split ring inprestressing contact with the surface of the countersink during axialmovement of the increasing diameter portion of the mandrel through saidring.
 12. The method of claim 11, comprising controlling the coefficientof friction of the lubricant to in that manner control the friction ofthe maximum diameter portion of the mandrel within the sleeve and hole.13. The method of claim 11, comprising controlling the length of themaximum diameter portion of the mandrel to in that manner control thefriction of the maximum diameter portion of the mandrel within thesleeve and hole.
 14. Apparatus for prestressing a countersunk fastenerhole, comprising:a mandrel for inserting into the hole for retractingtherefrom, said mandrel having a reduced diameter portion, an increasingdiameter portion axially outwardly of said reduced diameter portion, anda maximum diameter portion axially outwardly of said increaing diameterportion, said maximum portion having a slightly smaller diameter thanthe fastener hole diameter: an expandable disposable cold-expansionsleeve on the reduced diameter portion and insertable into the hole toserve as a spacer during the prestressing operation, for receivingexerted sliding expansion forces from said increasing diameter andmaximum diameter portions as the mandrel is retracted through the holewhile simultaneously carrying said forces in a radial direction to theinside surface of the hole for hole enlargement and for compression ofthe metal around the hole to improve fatigue properties of the metal;and an axially split, expandable ring member having a central passagewayfor receiving said mandrel and having a segmented annular portion sizedto fit into the countersink of the hole;said increasing diameter andmaximum diameter portions of the mandrel being adapted to be received insaid passageway and expand said ring member to force said annularportion radially outwardly against the countersink surface to compressthe metal of and around the countersink to improve the fatigueproperties of the metal in the region of the countersink.
 15. Apparatusaccording to claim 14 in which:the axial length of the maximum diameterportion is about equal to its diameter.
 16. Apparatus according to claim14 including:means for supporting the split ring member axially as it isbeing expanded and to support said sleeve axially.
 17. Apparatusaccording to claim 16 in which:said maximum diameter portion and saidannular portion are sized so that the radial distance of prestressingthe countersink is the same as that of the remainder of the fastenerhole.
 18. Apparatus for prestressing a fastener hole, comprising:amandrel for inserting into the hole and for retracting therefrom; saidmandrel having a reduced diameter portion, an increasing diameter and amaximum diameter portion respectively axially outwardly of said reduceddiameter portion; an axially split, expandable ring member having acentral passageway for receiving said mandrel, and having segmentedarcuate portions; said increasing diameter and maximum portions beingadapted to be received in said passageway and expand said ring member toforce said arcuate portions outwardly; said maximum diameter portionhaving an axial length about equal to its diameter.
 19. Apparatusaccording to claim 18 including:a disposable expandable sleeveinsertable into the hole, while positioned on the reduced diameterportion of the mandrel, to serve as a force transmitting spacer duringthe prestressing operation, for receiving exerted sliding expansionforces from increasing diameter and maximum portions as the mandrel isretracted through the hole while simultaneously carrying said forcesinto radial direction to the inside surface of the hole for holeenlargement and for compression of the metal around the hole to improvefatigue properties of the metal.
 20. Apparatus according to claim 19,including:a solid lubricant coating on the inner surface of theexpandable sleeve, providing the inner surface of the expandable sleevewith a predetermined coefficient of friction.
 21. Apparatus according toclaim 19 including:means for supporting the sleeve axially as said splitring member it is being expanded.
 22. In a pull gun of a type which isused for pulling a mandrel through a prestressing sleeve located withina fastener hole, so that an increasing diameter portion and an adjacentmaximum diameter portion of the mandrel can exert expansion forces onthe sleeve, to cause the sleeve to be expanded in the radial directionand prestress the metal around the fastener hole, to in that mannerimprove fatique properties of the metal,the improvement comprising: anexpandable member having axially split portions and having a generallycentral passageway through which the mandrel travels as it is beingretracted into the pull gun; said axially split portions being annularlyspaced around said passageway and normally being held radially towardthe center of said passageway; said passageway being normally smaller indiameter than the maximum diameter of the mandrel and a substantial partof the increasing diameter portion of the mandrel, so that as themandrel moves through such passageway it enlarges the passageway byforcing the split portions of the member apart; and said memberincluding means to engage a countersunk portion of said hole toprestress metal forming the same when said split portions are forcedapart in response to axial movement of the mandrel through saidpassageway.
 23. The invention according to claim 22 in which:said meansto engage are conical-shaped portions of said split portions, havingsubstantially the same angle as said countersunk portion.
 24. Theinvention according to claim 22 in which:said member having an inner endsecured within said gun and having an outer end extending outwardly ofsaid gun; said means to engage being on the outer end of said member;said means to engage are conical-shaped portions on said split portions,having angles complementary to the countersink angle.
 25. A method ofprestressing a countersunk fastener hole in a metal member,comprising:positioning a prelubricated cold-expansion sleeve on areduced diameter intermediate portion of a cold-expansion mandrel, saidintermediate portion being axially inwardly of an increasing diameterportion, a maximum diameter portion of said mandrel extending outwardlyof said increasing diameter portion; inserting the mandrel and sleeveinto the hole from its countersunk side; and holding the sleeve axiallyin the hole while retracting the mandrel through the sleeve and throughan axially split member having axially split arcuate prestressingportions projecting into the countersink, so that as the mandrel isretracted the increasing diameter and maximum portions of the mandrelexpand the sleeve radially to prestress the metal surrounding said hole,and expand the arcuate portions radially to prestress the metalsurrounding the countersink.
 26. The method of claim 25 in which:thesleeve is held in the hole with retaining means associated with saidmember and secured in a mandrel retracting pull gun.
 27. A methodaccording to claim 26 in which:said retaining means is axially split,having a central passageway through which the mandrel travels whilebeing retracted; and means on an inner portion of said mandrel forengaging said retaining means when the mandrel is in the fully extendedposition to position the retaining means so that it can engage thesleeve axially.
 28. Apparatus for prestressing a countersunk fastenerhole, comprising:a mandrel for inserting into the hole for retractingtherefrom, said mandrel having a reduced diameter portion, an increasingdiameter portion axially outwardly of said reduced diameter portion, anda maximum diameter portion axially outwardly of said increasing diameterportion; an expandable disposable cold-expansion sleeve on the reduceddiameter portion and insertable into the hole to serve as a spacerduring the prestressing operation, for receiving exerted slidingexpansion forces from said increasing diameter and maximum diameterportions as the mandrel is retracted through the hole whilesimultaneously carrying said forces in a radial direction to the insidesurface of the hole for hole enlargement and for prestressing of themetal around the hole to improve fatigue properties of the metal; and anaxially split, expandable member having a central passageway forreceiving said mandrel and having split arcuate portions sized to fitinto the countersink of the hole; said increasing diameter and maximumdiameter portions of the mandrel being adapted to be received in saidpassageway and expand said member to force said arcuate portionsradially outwardly against the countersink surface to prestress themetal of and around the countersink to improve the fatigue properties ofthe metal in the region of the countersink.
 29. Apparatus according toclaim 28 in which:the axial length of the maximum diameter portion isabout equal to its diameter.
 30. Apparatus according to claim 28including:means for supporting the split member axially as it is beingexpanded and to support said sleeve axially.
 31. Apparatus according toclaim 30 in which:said maximum diameter portion and said arcuateportions are sized so that the radial distance of prestressing thecountersink is the same as that of the remainder of the fastener hole.32. Apparatus for prestressing a fastener hole, comprising:a mandrel forinserting into the hole and for retracting therefrom; said mandrelhaving a reduced diameter portion, an increasing diameter and a maximumdiameter portion respectively axially outwardly of said reduced diameterportion; an axially split, expandable member having a central passagewayfor receiving said mandrel, and having split arcuate portions; saidincreasing diameter and maximum portions being adapted to be received insaid passageway and expand said member to force said arcuate portionsoutwardly; said maximum diameter portion having an axial length aboutequal to its diameter; said arcuate portions on said member having outerends sized to fit into a countersink of the hole and for being expandedwith said member to force said arcuate portions into the countersink toprestress the metal surrounding the countersink.
 33. Apparatus forprestressing a fastener hole, comprising:a mandrel for inserting intothe hole and for retracting therefrom; said mandrel having a reduceddiameter portion, an increasing diameter and a maximum diameter portionrespectively axially outwardly of said reduced diameter portion; anaxially split, expandable member having a central passageway forreceiving said mandrel, and having split arcuate portions; saidincreasing diameter and maximum portions being adapted to be received insaid passageway and expand said member to force said arcuate portionsoutwardly; said maximum diameter portion having an axial length aboutequal to its diameter; and a disposable expandable sleeve insertableinto the hole, while positioned on the reduced diameter portion of themandrel, to serve as a force transmitting spacer during the prestressingoperation, for receiving exerted sliding expansion forces from saidincreasing diameter and maximum portions as the mandrel is retractedthrough the hole while simultaneously carrying said forces in the radialdirection to the inside surface of the hole for hole enlargement and toprestress the metal around the hole to improve fatigue properties of themetal; said actuate portions on said member having outer ends sized tofit into a countersink of the hole and for being expanded with saidmember to force said arcuate portions into the countersink to prestressthe metal surrounding the countersink.
 34. Apparatus for prestressing afastener hole having a countersink, comprising:a mandrel for insertinginto the hole and for retracting therefrom; said mandrel having areduced diameter portion, an increasing diameter and a maximum diameterportion respectively axially outwardly of said reduced diameter portion;an axially split, expandable member having a central passageway forreceiving said mandrel, and having split arcuate portions; saidincreasing diameter and maximum portions being adapted to be received insaid passageway and expand said member to force said arcuate portionsoutwardly into said countersink; said maximum diameter portion having anaxial length about equal to its diameter to insure that the arcuateportions remain in the countersink after the member has been expanded toprestress the metal surrounding the countersink.